On-line voltage stability monitoring for electric power system Part 1: On-line voltage stability index considering load static characteristics

Based on the load static characteristics and local measurements, a new index for on-line static voltage stability analysis is proposed. The ZIP load model is introduced and the equivalent line current is divided into real and imaginry parts based on Thevenin equivalent circuit. After eliminating the active power and substituting the reactive power of ZIP load for the imaginary part, a quadratic is obtained,which is processed by the root discriminant to deduce a voltage stability index VSIQ. Similarly,eliminating the reactive power, VSIP is derived. The smaller of VSIQ and VSIP becomes the bus voltage stability index VSI and the bus with the smallest VSI is the weakest one of system. The proposed index is tested on IEEE 14-bus and IEEE 30-bus systems, and the simulative results indicate that,the more the percentage of constant power is, the smaller the VSI will be. When the percentage of constant power is identical, the more the percentage of constant current is, the smaller the VSI will be.